Classic is the highest storage capacity iPod. It has, for the past few years, come in 120GB and 160GB versions, with few differences between the models (some were slightly thicker, some were slightly thinner, but not major changes).
It does not have many of the same features as the touch, however. It has a smaller screen, only plays/displays media (music/video/photos), does not have a touch screen, etc. It's an entirely different iPod from the touch - if you want extremely high capacity and just media playback, then it's a good choice. But if you like the touch for the appstore, the calendar/contact apps, the ability to surf the internet... The classic would not be an appropriate replacement.
Your other option is to acquire a larger-capacity touch - though this may be prohibitively expensive, depending on your situation. The touch has come, to date, in 8, 16, 32, and 64 GB capacities (with the 64 GB just introduced three days ago). I suppose there's also the 16GB nano, but you may fill that up too quickly to be a real upgrade - and it has the same limitations as the classic, but with less storage space.
